Switching circuit



mais e Ncws WAHM/VWM 7% WW ATTORN EY Feb. 5, 1952 c. P. cooPER ETAL swITcHING CIRCUIT Filed April s.' 1947 Patented Feb. 5, 1952 SWITCHING CIRCUIT Charles Peter Cooper, Chelmsford, and Guy Edward Williamson-Noble, London, England, assignors, by mesne assignments, to Radio Corporation of America, New York, N. Y.; a corporation of Delaware Application April 8, 1947, Serial No. 7 40,222 In Great BritainuDecember 18, 1945 Section 1, Public Law 690, August 8, 1946 Patent expires December 18, 1965 4 Claims.

This invention relates to voice Vand like signal operated switching circuit arrangements and, though not limited to its application thereto it is particularly adapted for use in radio telephone systems for automatically switching on vthe carrier for speech and switching it off when speech ceases. Such automatic switching is frequently required for example in duplex systems where the same carrier Wave. length is employed for speech in both directions.

The practical requirements to be satisiied in such a voice operated switching system are very onerous. Firstly it must be very quick acting for if the carrier be not fully switched on by the initial consonant of the first word clipping of the speech will occur at each switching on especially if the consonant is of low energy content. In practice this requirement involves that an audio signal about 30 db, below that required for full modulation should bring the transmitter fully on the air not more than 2 milli-seconds after said signal reaches the modulation circuits. 'Ihe common expedient of meeting this diiiiculty by .inserting a delay line between the audio input and the modulation circuits is objectionable on the ground of expense and, on this ground, a quick acting relay is much to be preferred.v

A second diiculty is that of noise. Noise outside the speech band or other band of audio-frequencies to be handled can be eliminated by means of a iilter but noise including background noise in the microphone room must be below the level required for switch operation. This involves that if sensitivity be increased too much the transmitter may be switched on by noise which may further hold the carrier on after speech has ceased, whereas it is essential in most systems, such as a duplex system with common Acarrier frequency for two-Way working, that release should occur-i. e. the carrier be switched cil-- on cessation of speech. In practice a difference of only 2 db is usually required between operation and release and considerable dimculty is experienced, in known arrangements, in securing reliable operation with so small a difference of level.

If a carrier is switched fully on in 2 milliseconds or less, a light click is heard on the receiver headphones and it is therefore desirable to reduce the number of switchings as far as possible. For this reason a so-called hang-over circuit, giving a hang-over time of about 0.1 second is usually required to hold the carrier on for complete phrases of speech as distinct from allow ing release in the short inter-syllable and inter- `wordv pauses of ordinary speech. Ideally this l hang-.over time should be independent of the level 'qpolarityof the half-waves. U Preferably la ydelay and duration of aspeech phrase and in practice a variation of about 1.5 to l is allj that is perf missible. f l

The hang-over circuit will normally havel a logarithmic law of voltage decay and accordingly if it be appliedk direct to the transmitter control circuits a transient condition will arise when the transmitter is nearly oii,l the normal humor noise on the carrier then giving a high degree of modulation causing a whistle or grunt onthe receiver signal. The hang-over circuit therefore normally operates by switching ofi the carrier at the end of the hang-over period through a relay provided for the purpose-afurther complication.

In many cases there is the further requirement that a local receiver has to be rendered insensitive just before the carrier comes on and rendered sensitive again after the carrier has .c gone 01T. This involves the provision of another set of circuits if the transmitter is controlled Aby an elec,- tronic type of switch. l j. `j

The present invention seeks to provide improved and reliableswitching circuit arrangefments capable of satisfying the practical requirements witha minimum of complication and op'- erating through a high speed switchpreferably a high speedelectro-magnetic relay-to perform the necessary transmitter (and, if required, local receiver) circuit changes, kthe switching circuit arrangements provided by the said invention securing relay or switch change over in the required short time of about 2 milli-seconds or less.

According to this invention a voice or like signal operated switching circuitl arrangement comprises a high speed switch or relay controlled by the output current of a controlling electron discharge tube circuit; a limitedl pulse providing .electron discharge tube circuit connected with the controlling `circuit to cause it to producel a ypredeterminedchanged output current (preferably zero) in response to a pulse, said pulse producing electron discharge tube circuit having two viding input to said pulse producing circuit to cause it to produce a pulse in response to each half wave of actuating signal input above apredetermined minimum level irrespective of `the circuit adapted, in discharged condition, to hold the controlling circuit `near said'fpre-determined output current between pulses is provided, "said delay circuit being arranged to be discharged by the iirst pulse as rapidly as possible.

As is well known, practically any transmitter can be arranged to be switched on by 'earthing one point in its keying circuitandfa receiver can also usually be arranged to be rendered sensitive by earthing one point thereof. `In -the 'preferred way of applying the inventicn,'therefore,'the high speed switch or relay is used to put on and take oi these earth connections: by employing a suitable electro-magnetic relay with an'earthed tongue both a transmitter and local receiver may be controlled by the same relay.

The invention is illustrated in the accompanying drawing which shows diagrammatically a preferred embodiment thereof.

Referring to the drawing, audio input is applied through a vtransformer TI and adjustable potentiometer PI between lthe lcontrol grid and cathode of a 'tube VI, shown as a pentode, the movable point on the potentiometer being connected to the grid and its .ends being connected across the secondary of the transformer TI. vOne end of this secondary yis connected .through la condenser CI to the cathode and through a re Vsistance R2 to a junctionpoint with a resistance R3`who`se other en'd is connected'to vthe cathode.

YThis junction'pont is taken to earth through a resistance'Rl vand 'through a condenser C3 to the suppressor grid of a pentode V3, said supvpressor'grid being connected to `earth through a resistance R1. `YThe screen and suppressor grids of tube VI are strapped to its anode which is taken'through a resistance R5 to a sourceof high tension potential (not shown) connected at HT-t. 'This anode is also connected through a condenser "C2 tothe'suppressor of a pentode V2, said suppressor 'being connected to earth through a resistance RB. The cathodes of tubes V2 and V3 are connected together and to their respective control grids; the'screen gridsare 'connected together and through a resistance R9 to HT+z and 'the anodes are connected together and through 'a resistance R8 to `HI'+, the common anode 'connection being taken `through a resist- 'ance'RI in parallel'with a condenserCl to the control grid of a pentode'Vl, said grid being connected through a resistance RI I to earth and to the negative terminal `HT of the high tension supply. Thescreen grid of tube V4 receives potential'from HT-I- 'through Va resistance RIZ and isconnected' to earth 'through condenser C5 while its anode is connected directly to HT+ and its 'suppressorgrid is Vstrapped to the cathode, the

said 'cathode being connected directly to that of 'a vi'urther pentode V5 and. through two resist- Ia'nces `RI3 `and 'RH 'in series to ground. 'The 'cathode 'of V3 is `connected tothejjunction 'point of resistancesRH and RH. 'The Vanocle'and all three `grids of V5 `are connected 'together and to earth through `a condenser C8 and, through resistan'cesRIS andRII (the latterpreferably adjustable) tothe cathode of a pentode V3. This cathode is connected 'to earth through a resistance RIB which is preferably adjustable. A condenser C'I is vin'parallel with'resistance RIB.

'The `control grid of V8 receives input directly :from the 'common grid-anode point of V5 and itsscreengrid is connected 'to HT-l-'its suppressor grid'be'ing connected to the Vcathode and its anode being connected 4to HT+ through the winding SI oi 'a high speed electro-magnetic relay hav- 4 ing an earthed tongue T and "mark and "space" contacte M and/S. Thefwual bypals'condenser C8 is connected across tberHT.

In the absence of input Vl passes a large anode 'current and V3 a small one but i! the suppressor -current of V4 is thus rapidly cut off and that of V3 increased at the expense of screen current. The speed of this change over is increased, by the provision of condenser Cl which reduces the eiect ofstray capacity in the grid circuit of V4. vWhen .the suppressor of V3 lialls 'to a certain value below that required for initiation of the foregoing action the sequence is reversed and the cathode-coupled multivibrator circuit constituted by the circuitsof V3 and Vl reverts to the original state. Thus a sine wave input 'to the suppressor of V3 will produce across RI3 and RII rectangular waves of substantially constant amplitude so long as the input is sumcient to drive Vl from the grid current condition. If it'is insumcient there is virtually no output. As so far described an initial positive swing on the suppressor oi V3 will operate the multivibrator but an initial negative swing will not. It is for-thisreason. that is to say to vensure rapid triggering action whatever the polarity of the initial half wave, that the tube V2 is provided, this tube being connected in practically the same manner as V3 acting similarly on V4 except that, owing to the phase splitting action of VI. when the input to V3 is negative that to V2 is positive. Its provision modifies'somewhat the waveshape of the output from VI but not so as to upset the operation of the delay circuit which, as will be 'apparent later, requires only that the maximum negative excursion of the wave shall be independent of`the input to the combination V! V3 YVl which, as will now be seen, acts as what may be termed a limiter multivibrator.

The delay circuit. comprises V5, which acts as a diode in a resistance-capacity network. When the voltage across RI3 and RI 4 suddenly reduces. C6 is rapidly discharged through V5, RI3 and RM, thus reducing the anode current of V6. This operates the relay to close the mark contacts. C1, by preventing 'rapid variations in cathode voltage of VB'speeds up this fall of anode current resulting in about a 3 to 1 increase in operating speed without materially laffecting release (movement to space") speed. When the input to the multivibrator limiter falls below the threshold, the cathode of V5 returns to normal and this tube ceases to conduct, C8 thenre-charging through RIB andRI1 from the voltage developed across RIB. As C6 re-chargeathe ,grid voltage andanode current'of V6 increases, causing an increase nearly equal to the grid voltage increase in the voltage across RIIi, 4thereby keeping almost constant voltage across RIS and RI1. This effect increasesby'a factor 0f nearly l0 to 1 the time taken by C6 to recharge toa given voltage, thereby allowing'for a 'given release time, the use'of a condenser'C of a value of aboutone tenth that which would be required were it charged 'from Y,a constant 'voltage'source through the 'same value of resistanees RIB RIT. yIt follows, accordingly, that discharge time-constant (and consequentlyoperating time) is reduced in about the same ratio. When the anode current of V6 reachesa certain value the relay operates to open the mark contacts yat a time interval (after the cessation of audio input) which is controlled by adjustment of RIT. Shortly after this the voltage across C6 reaches a value which allows V5 to conduct thereby preventing further increase in grid voltage and anode current of V6. Ri is used to set the current through the relay in absence of input signal to a value just above that required to make the relay operate. This current varies slightly from relay to relay being dependent on the gap in the iron circuit and stiffness of springs. A fine setting is desirable to enable the relay to respond very rapidlyk by opening when the current through its coil starts to fall. An illustrative set of values of the diiierent electrical components are:

The relay operates with a current of about 8 ma. The H. T. supply is 300 volts.

The embodiment described has many important advantages: I'he use of the relay allows both transmitter and local receiver to be controlled by one set of circuits and renders the apparatus applicable to any normal telegraph-telephone transmitter; the time to operate can be made under 2 mini-seconds and is almost independent of input level once a certain minimum is exceeded; a very small fall in level below minimum causes a release; the hangover time (given by the delay circuit) is almost independent of input after a iirst period equal in practice to a maximum of about 10 milli-seconds; no relay specially provided to switch oil' the transmitter alone is required; and no large preampliiication or high performance A. V. C. chain is required so that stable operation from a low cost, low consumption small i installation is obtainable. In fact, in an experimental installation a tube consumption of only about 30 ma. at 200 volts with an overall size of only 4" x 11 x 5" (including input transformer, input level potentiometer and filter to cut off frequencies below 700 cycles per second) have been attained with an ability to operate from -36 db on 1 mw. corresponding to a full modulation level of -6 db on 1 mw.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed we declare that what we claim is:

1. A voice operated switching system comprising a source of message signals providing a waveform having positive and negative half waves, a phase splitting circuit coupled to said source and comprising an electron discharge device having an anode, a cathode and a grid, a transformer coupling said source to said grid and cathode, a limiter comprising rst, second and third electron tubes each including a grid, a cathode and an anode, a connection between the anode of said device and the grid of said first tube, a connection from the cathode of said device to the grid of said second tube, direct connections'between the cathodes of said first and second tubes,

direct current connections including resistors respectively `extending from'the cathodes of said device and said third tube tothe cathodes of said first and second tubes, a resistor shunted by a condenser connected between the anodes of said first and second tubes and the grid of saidthird tube, whereby said limiter produces a pulse in response to each half wave of the waveform applied by said source to said device about a predetermined minimum irrespective of the polarity of the half waves, a fourth vacuum tube coupled to the output of said third tube and controlled by the pulses produced by said limiter, and a load device controlled by the output current of said fourth tube.

2. A voice operated switching system comprising a source of message signals providing a waveform having positive and negative half waves, a phase splitting circuit coupled to said source and comprising an electron discharge device having an anode, a cathode and a grid, a transformer coupling said source to said grid and cathode, a limiter comprising first, second and third electron tubes each including a grid, a cathode and an anode, a connection between the anode of said device and the grid of said first tube, a connection from the cathode of said device to the grid of said second tube, direct connections between the cathodes of said first and second tubes, direct current connections including resistors respectively extending from the cathodes of said device and said third tube to the cathodes of said first and second tubes, a resistor shunted by a condenser connected between the anodes of said iirst and second tubes and the grid of said third tube, whereby said limiter produces a pulse in response to each half wave of the waveform applied by said source to said device above a predetermined minimum irrespective of the polarity of the half waves, a fourth vacuum tube coupled to the output of said third tube and controlled by the pulses produced by said limiter', a delay circuit between the output of said limiter and the input of said fourth vacuum tube, and a load device controlled by the output current of said fourth tube.

3. A voice or like signal operated switching system, comprising a source of signals providing a Waveform having positive and negative half waves, a phase splitting circuit coupled to and fed by said source and providing an output half wave in response to each half wave of signal from said source, said phase splitting circuit comprising an electron discharge device having two output terminals at which appear output half waves of opposite phase in response to each half wave of signal from said source, an electron discharge device cathode-coupled multivibrator circuit having two conditions of electrical stability coupled to the output of said phase splitting circuit and providing pulses of substantially constant amplitude independent of the amplitude of the half waves supplied thereto by said phase splitting circuit above a predetermined minimum level, an electron discharge tube circuit coupled to the output of said multivibrator circuit, said tube circuit being acted on by said pulses to produce therein, in response to said pulses, a predetermined changed output current, and a load device coupled to thevoutput of said tube circuit and responsive to said predetermined changed output current.

4. A voice or like signal operated switching Vfed by saidv source and providing an output half wave in response to each half wave of signal from said source, said phasesplitting circuit corn- .prisingan electron discharge device having two output terminals vat which appear output half waves of opposite phase in response to each half Awave ofl signalfrom said source, an electron discharge device cathode-coupled multivibrator circuit having two conditions of electricalstability coupled to the output of said phase splitting circuit and providing pulses of substantially constant amplitude independent of the amplitude of the half vwaves supplied thereto by said phase splitting circuit above a predetermined minimum leveL-an electron discharge tube circuit coupled to the output of said multivibrator circuit, a delay circuit positioned between said multivibrator vand said tube circuit, said tube circuitbeing acted on by said pulses to produce therein, in

' response to said pulses, apredeterminedchanged output current, and a load device coupled to the output of said vtube circuit and responsiveto nid predetermined changed output current. `CHARLES PETER COOPER.

GUY EDWARD WILLIAMSON-NOBLE.

REFERENCES `CITED The following references are of yrecord in the le of thispatent:

UNITED' STATES PA'I'ENTS Number Name Date 1,636,031 Wright July 19, 1927 1,818,018 Stocker Aug. .11, 1931 2,018,464 Nebel Oct..22, 1935 2,083,666 Potter June 15, 1937 2,102,903 Leveque Dec. 21, 1937 2,157,234 Hesner May 9, 1939 .2,194,559 Koch Mar. 26, 1940 2,251,276 Fisher ,Aug 5, 1941 V2,267,622 Mitchell Dec. 23, 1941 FOREIGN PATENTS Number Country Date l529,368 Germany July l1,- 1931 

